1. Field of the Invention
This invention relates to apparatus and systems for efficient and effective purification of water to provide improved pressure drop characteristics and capacity as compared to purification systems using conventional carbon blocks.
2. Description of Related Art
Water filtration media containing extruded or molded carbon blocks (particles of granulated activated carbon held together formed into a desired shape and held together with a polymeric binder) have been known in the art. For instance, U.S. Pat. Nos. 5,189,092; 5,249,948; and 5,331,037 disclose carbon blocks used to purify water prepared by extruding carbon particles with a polymeric resin.
These conventional carbon blocks, while functional, present several disadvantages in use. First, the capacity of the blocks is limited by the kinetics of adsorption onto the carbon surfaces and into the pores of the bound carbon particles. Because so much of the surface area of the carbon particles is occluded by binder resin, the capacity of filters of reasonable size and number to treat water is rather limited in terms of volumetric flow rate. In addition, the total volume of water treated is limited because of clogging of the filter due to the presence of small particulates and other detritus in the water being filtered, as well as because of the saturation of the limited number of active sites on the carbon particulate.
Second, both the volumetric flow rate and the total volume of water treated can be limited by the structural integrity of the carbon block. Many conventional carbon blocks degrade over a fairly short period of time as the flow of water erodes them, breaking them into smaller blocks that can move around in the flow system and cause clogging.
Third, carbon blocks, in particular extruded carbon blocks, can be difficult to manufacture with high consistency, leading to unpredictable performance results. The binder resins must be heated, either directly or by shear in an extruder, which can passivate some of the carbon, rendering the blocks less effective.
Fourth, existing carbon block formulations are not particularly effective at removing chloramines, rendering them less than desirable for use in water purification systems used in the food processing industry. In particular, catalytic carbonaceous chars can be used for chloramine removal, as disclosed in U.S. Pat. No. 6,342,129, the entire contents of which are incorporated herein by reference. However, the inventor has found that including this material in a carbon block results in a substantial amount of deactivation of the catalytic activity of the carbon, believed to result from the need to heat the binder resin to form the carbon block.
Moreover, chloramine reduction is very complex to achieve by carbon block filters made from regular granulated activated carbon, be it molded or extruded, etc. For example, extruded carbon block can remove chlorine with one pound of highly activated coconut shell carbon at a flow rate of approximately 2 gpm with 80×325 mesh carbon particles and 15% polymeric binder for approximately 7,000 gallons to meet NSF Standard 42 at 2 ppm influent chlorine and 0.5 ppm effluent. Where the same filter needs to reduce chloramine at 2 ppm, its capacity is barely 200 gallons at the same flow rate.
Special catalytic carbons have been developed to remove chloramine, but when they have been molded or extruded into carbon block, they lose 90% of their efficiency for chloramines removal. Different carbons have different adsorption capacities; however, coconut shell carbon, coal based carbon and other nut-derivative materials all demonstrate a ratio of 20:1 or 30:1 capacity between their ability to remove chlorine and their ability to reduce chloramine.
Water pH is also a factor in the effectiveness of the chloramines removal media. The pH of chloramine in water is extremely high, generally between about 8.5 and about 9.6. At this pH level, most carbon loses its removal efficiency quite rapidly, if not immediately. Since a water filter requires a minimum amount of water flow to function effectively and economically, the size of the filter becomes critical. With regular carbon, to achieve chloramine reduction at the same capacity as chlorine with the same filter, a sufficiently long contact time is needed such that the filter must be about 10 to about 20 times larger than is necessary if only chlorine is to be removed.
Carbon block filtration results in other disadvantages as well. It is often difficult to prepare carbon blocks with adequate control over the particle size of the particulate carbon used to make the blocks. This can result in the use of carbon containing significant quantities of dust, which can migrate through the carbon block, clog pores, and create unduly large pressure drops through the block, which can block as much as 50% to 90% of the rated flow rate through the filter block. Plugging of the filter also creates inconsistencies in water flow. Carbon made from coconut shell, in particular, can swell in the presence of VOC, further constricting fluid flow and increasing pressure drop.
An alternative to carbon block technology is the use of granulated carbon and other filtration media, generally having a particle size distribution ranging from about 50 mesh to about 28 mesh, in a canister through which the water to be purified flows. Again, while this arrangement is functional, particularly when coupled with a downstream carbon block purification step, the capacity of the system is generally limited to around 6,000 to 7,000 gallons.
Accordingly, there remains a need in the art for a water purification apparatus and system that does not suffer from the disadvantages of carbon block based systems, that can purify water at high capacity, and that can be adapted to effectively remove chloramines from water. In addition, there remains a need for an essentially modular system, which provides effective water purification through the use of a single cartridge, yet which provides sufficient flexibility in manufacture that different filtration media can be used without multiplying the inventory of parts needed to produce the system.